Contents for Frequency and Periods of Seismic Waves

• Introduction
• Activity 1: Using WinQuake to View a Seismogram
• Activity 2: Calculating Frequency
• Activity 3: Using Changes in Frequency to Pick S-wave Arrivals
• Activity 4: Seismic Waves and Sound

Introduction

In traditional physics texts, nearly every example of a wave is a pure sinusoidal wave. Sometimes, sinusoidal waves are superimposed to form more complex, but still periodic or repeating, waves. Students can develop the misconception that all waves are simple, and that the frequency pattern of any wave will be repeated regularly over time and space. In nature, there are many waves that are not periodic, and seismograms are good examples. Representing mechanical motion easily imagined by students, seismograms may be a useful introduction to more abstract complex waves, such as changing electromagnetic and sound waves.

The motions of the earth at the hypocenter of an earthquake are irregular and non-repeating, and create deformations in the surrounding rock that spread through the earth. Different phases of the motion (such as compressional and shear phases) travel at different speeds, and the motion of the ground at the site of a distant seismometer is complex and changes over time. Changes in the frequency of the earth's motion can be seen in seismograms as wide or narrow peaks and troughs (with longer period and lower frequency, or shorter wavelength and higher frequency).

A single seismogram displays seismic waves at a location (the recording station) over time. A seismogram is not a snapshot. Rather you could think of it as a movie of the waves that traveled through this location.

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Activity 1: Using WinQuake to View a Seismogram

Use WinQuake to look at an interesting seismogram (of a local, recent, or famous event). Students may describe how the seismogram changes appearance along the time axis, discuss how the earth must have been moving as time passed, and then phrase their descriptions in terms of frequencies. A seismogram which shows displacement rather than velocity will provide students with the most easily visualized depiction of the earth's motion.

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Activity 2: Calculating Frequency

Also, have students determine the period of the seismic wave at various times. Then they may calculate the wave's frequency at each of these times in the standard units of Hertz.

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Activity 3: Using Changes in Frequency to Pick S-wave Arrivals

Changes in the frequency of ground motion can be used in identifying S-wave (shear) arrival times. S waves have lower frequencies than P (pressure, or compressional) waves, and this decrease in frequency is often visible on a seismogram. Have students practice picking S waves on available seismograms in this way.

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Activity 4: Seismic Waves and Sound

Remind students that P waves are sound waves. After students calculate the frequency of a P wave in Hertz, have them compare the P wave to sound waves they can hear. They may wish to discuss whether it might ever be possible to hear a P wave generated by an earthquake; whether some animals may hear P waves; and whether P waves would be loud enough to hear, at various distances.

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